A typical hydraulic construction machine such as a hydraulic excavator includes a pump torque control system having a regulator, which controls a displacement volume of a hydraulic pump and has a pump torque control function. The displacement volume of the hydraulic pump is controlled by the pump torque control system to ensure that an absorption torque of the hydraulic pump does not exceed a preset maximum absorption torque. This suppresses overload applied to a prime mover and prevents an engine stall.
In connection with such a pump torque control system for a hydraulic construction machine, Patent Document 1 discloses a control method titled “Method for controlling drive system including internal combustion and hydraulic pump”. The control method is to obtain the difference (engine speed deviation) between a target engine speed and an actual engine speed detected by an engine speed sensor and control an input torque of a hydraulic pump based on the engine speed deviation. The control method is an example of speed sensing control. This speed sensing control is capable of temporarily reducing the maximum absorption torque during the pump torque control, reliably preventing an engine stall due to overload applied to a prime mover, and quickly increasing an engine speed by controlling the amount of fuel to be injected.
In addition, in connection with such a pump torque control system, Patent Document 2 discloses a technique for controlling the maximum absorption torque of a hydraulic pump based on sensed environment related to a prime mover and the periphery of the prime mover, and suppressing a reduction in the engine speed of a prime mover even when power output from the prime mover is reduced due to a change in environment.    Patent Document 1: JP-B-62-8618    Patent Document 2: JP-A-11-101183